There is a need for covalent modification of polymer surfaces to be used as carriers of molecules, as support for cells or as sorbents in separation methods. The limitations of adsorptive coating of polystyrene, herein referred to as PS, which can be used, for example, as microtiter plates or for bio-assays, are generally known and the need for covalent coupling methods is recognized in the art.
Functionalized PS suitable for covalent attachment of ligands and antibodies has been obtained by the following three methods: copolymerization of monomers containing functional groups as disclosed in Lloyd and Durscher, J. Appl. Polymer. Sci., Vol. 7, 1963, pp. 2025-2037; chemical modification of PS products as taught by Chu and Tarcha, P. J., J. Appl. Polymer. Sci., Vol. 34, 1987, pp. 1912-1924, and graft-polymerization of unsaturated monomers as demonstrated in EP 0 106 169. One problem with these procedures is that the resulting products have impaired optical qualities and/or high background noise in binding assays.
Only the last method involving graft-polymerization of unsaturated monomers uses radiation to initiate covalent bond formation between the support material and the spacer or molecules to be immobilized. Radiation graft-polmerization uses polymerizable monomers to build up a chain in a radiation-initiated or free radical polymerization reaction. The disadvantages associated with radiation graft-polymerization are as follows: 1) Graft-polymerization is technically difficult to do and it is not easy to control, i.e., the length of the grafted polymer is variable; 2) During the process of graft-polymerization other reactions, such as cross-linking of the grafted polymer chain may take place in addition to polymerization resulting in a rigid radiation-grafted layer of polymer; 3) In cases when the ligating group is attached only to the end of the grafted chain, graft-polymerization is not expected to yield high capacity of support; 4) Since in ligandantibody interactions approximately 10 .ANG. length spacers have been found optimal, See Lowe et al., Biochem. J., Vol. 133, 1973, pp. 499-514, the long (hundreds to thousands of monomer-length) grafted chains obtained in EP 0 106 769 are suboptimal; 5) In most graft-polymerization methods large areas of charged and/or hydrophobic surfaces are introduced, leading to high levels of nonspecific interactions and unacceptable background noise, See, Lowe and Dean, P.D.G. Affinity Chromatography, J. Wiley and Sons, N.Y. 1974. These disadvantages of radiation graft-polymerization can be circumvented by using nonpolymerizable molecules for radiation-mediated derivatization of polymers as described below.